Field of the Invention
This invention relates generally to a method for switching between a full winding control mode and a half winding control mode for a three-phase machine and, more particularly, to a method for switching between a full winding control mode and a half winding control mode for a three-phase permanent magnet (PM) AC electric machine including split stator windings so as to reduce back EMF and increase the torque and power of the machine at higher machine speeds.
Discussion of the Related Art
An electric machine having a wide speed range is essential for automotive propulsion systems, such as for hybrid vehicles, electric vehicles, fuel cell vehicles, etc., and for power generation applications. In order to maximize its torque/ampere ratio, an electric machine is typically designed to have as high of an induced voltage-to-speed ratio as possible. However, because the induced voltage is proportional, especially as the speed of the machine increases, the back electro-motive force (EMF) generated by the machine also increases as the machine speed increases until it reaches the DC bus voltage, generally a battery voltage, which results in a loss of EMF available to drive the current in the motor, which acts to limit the speed of the machine.
It is known in the art to separate the stator windings for each phase of an electric machine into two split windings to reduce machine back EMF at high machine speed. Switches are provided and are controlled so that the split windings for each phase are electrically coupled in series for low machine speeds and are electrically coupled in parallel when the speed of the machine reaches the point where the back EMF reduces the machine torque. However, by providing twice as many windings in the stator and the switches necessary to switch between an electrical series configuration and a parallel configuration, this solution for winding reconfiguration increases the number of required AC switches to nine and the total number of machine leads to ten for a three-phase machine. Further, there is the potential for circulating currents in the parallel configuration due to coil EMF mismatches. Also, coils are required to be in the same stator slot for parallel operation, and lower coil inductance in the parallel operation may need higher switching frequencies to reduce current ripple.
U.S. Patent Application Publication No. 2014/0239876 to Hao et al., published Aug. 28, 2014, assigned to the assignee of this application, discloses an electric drive system for a three-phase PM electric machine, where each phase of the machine includes a stator winding separated into a first winding section and a second winding section and two switches in an inverter electrically coupled to the winding sections. The drive system includes a switch assembly for each phase electrically coupled to the inverter switches and the first and second winding sections, where the switch assembly includes at least two switch states. A first switch state of the switch assembly electrically couples the first winding section and the second winding section in series to the inverter switches and a second switch state electrically coupled to the second winding section to the inverter switches and electrically disconnects the first winding section from the inverter switches.